This invention relates to vacuum pumps and is particularly directed to oil free mechanical vacuum pumps.
In order to improve the vacuum performance of such pumps, it is known to arrange a number of pumping stages in tandem and to include at the output stage and the stage immediately adjacent the output stage, rotors of intermeshing "claw" type.
In our United Kingdom Pat. No. 2111126, there is disclosed an oil free mechanical vacuum pump in which at least two pairs of rotors of the intermeshing "claw" type, are arranged in tandem in adjacent pumping chambers. In the pump disclosed and claimed in this United Kingdom Patent, each pair of rotors in a chamber are mounted on shafts driven to rotate in opposite angular directions, with the pairs of rotors in adjacent chambers being mounted upon the shafts in reverse orientation.
Such a pump construction has the advantage that by arranging the pairs of rotors in adjacent stages in reverse orientation, direct transfer of gas from one stage to the next through a transfer port in the partition walls separating adjacent stages, can occur with minimal interstage volume. The outlet of one stage on one side of the interstage partition, accordingly becomes the inlet of the next stage on the other side of the partition.
Conventionally, oil free mechanical pumps particularly of the type disclosed in United Kingdom Patent, No. 2,1111,26, have the rotor supporting shafts driven by suitably coupling one of the shafts, at one end, to a prime mover such as an electric motor and coupling the shafts, conveniently at an opposite end, through a gear train effective to produce rotation of the shafts in opposite angular directions.
A typical conventional shaft drive arrangement for such pumps is illustrated in FIG. 1 of the accompanying drawings. FIG. 1 illustrates two shafts, 2,4, of an oil free mechanical pump, driving pairs of "Claw" type rotors 6,8, supported upon the shafts in reverse orientation.
Extended drive shaft 2 is coupled to a prime mover such as an electric motor in any suitable manner (not shown), while drive shafts 2, 4 are coupled at the opposite end through engaging gears, 10,12 to provide rotation in opposite angular directions.
The gears 10,12 are correctly engaged for proper synchronization of the pairs of rotors 6,8 to ensure that the rotors do not touch in operation.
A number of disadvantages have been found to arise from the use of the gear train arrangement disclosed in FIG. 1:
Firstly, gears require lubrication to maintain efficient power transfer and to reduce to a minimum wear and consequent loss of synchronisation resulting from wear. The presence of oil is most detrimental in oil free mechanical pumps and costly oil seals, which require regular and expensive maintenance, are necessary to prevent oil transfer into the pumping chambers.
Secondly, most metal gear trains capable of transmitting the torque necessary to drive mechanical vacuum pumps, are in general very noisy and gear noise added to the noise of pump operation, often exceeds both tolerable and indeed statutory noise thresholds.